1. Field of the Invention
This invention relates in general to semiconductor devices and manufacturing methods thereof, and in particular to silicon semiconductor devices, and manufacturing methods thereof, in which such devices are provided with extremely flat, and thin, polysilicon overcoats.
2. Description Relative to the Prior Art
A typical solid state imaging device comprises a suitably doped wafer (chip), a silicon dioxide coating on the wafer, and transparent electrodes overlaying the silicon dioxide coating. It frequently happens that the transparent electrodes are formed of doped polycrystalline silicon (polysilicon) which is intrinsically compatible with silicon device manufacture.
Prior procedures for forming polysilicon electrodes have usually taken the tack shown in FIG. 1. As indicated, a silicon wafer having a coating of silicon dioxide thereon is exposed to reactant gases (at an elevated temperature) which blow past the wafer and, in so doing, cause a polysilicon coating to form on the silicon dioxide coating. (Shaping and otherwise forming electrode patterns, say by polysilicon etching, form no part of the invention, and are well known techniques in the art.)
Though exaggerated, FIG. 1 shows that the gaseous upstream side of the silicon wafer gets more thickly coated with polysilicon than does the downstream side, causing the polysilicon coating to taper in thickness. Such tapering has certain disadvantages: (1) since imaging devices are, inherently, optical devices, tapering causes "variable" fringing effects to occur across the face of the device; (2) variable sensitivity,, both monochromatic and spectral, across the plane of the device, i.e., from pixel to pixel. Also, even ignoring the tapering effects, chemical vapor deposition, as appears in FIG. 1, is not a tightly controllable practice, and results in batch-to-batch polysilicon thickness variations.
To obviate the tapering effect, and to produce consistently polysilicon layers that are both extremely, and uniformly, thin, say on the order of 1000 A.+-.20 `A, vacuum deposition of polysilicon was tried. Vacuum deposition of a coating, as is well known, is a generally well controlled procedure, and productive of coatings within close thickness tolerances: Source material (the evaporant) to be vacuum deposited is heated in a vacuum chamber, causing a vapor of the evaporant to deposit on the substrate being coated; a monitor extremely accurately registers the amount of deposition and, when the desired coating thickness has been noted, a shutter or the like is actuated to shield the structure being coated from the vapor.